A method of this type is already known from EP-B1-0 427 099 (W. Muller). It achieves the purpose of obtaining a predetermined degree of thickening of the retentate in an operationally assured and simple manner. This micro-filtration method, in which the material stream to be filtered passes under pressure in a circuit through a micro-filtration module with membranes, over which the stream passes tangentially with continuous addition and removal of concentrate, operates by measuring the velocity and pressure drop in the retentate stream. The concentration of a turbid material is calculated from these measured values and the concentrate output is adjusted in case of deviations from a set value.
The aim in connection with such membrane filtrations in the form of ultra-filtration or micro-filtration is to thicken the retentate in the course of the thickening process to obtain the highest possible proportion of wet sludge. By means of this it is possible to reduce the costs for subsequent drying and disposal of the retentate. The degree of retentate thickening which can be achieved depends essentially on the highest viscosity at which the retentate can still be passed through the membrane modules of the installation used. To achieve high viscosity, it has been known up to now to keep at low levels the number of modules in each series (during each pass) and the velocity of the retentate over the membranes. However, a reduction of the number of modules per pass reduces the output of the installation and thus decreases it efficiency.